1. Field of the Invention
The present invention relates to a method for estimating resistance values of a stator and a rotor of an induction motor, and more particularly, to a method for estimating, accurately and in real time, the change of resistance values of a stator and a rotor of an induction motor due to an increase in temperature during operation of the induction motor.
2. Description of the Related Art
The speed controlling method of an induction motor can be a voltage controlling type, a frequency controlling type, or an electromagnetic coupling type. The voltage controlling type method controls the speed of an induction motor by changing the voltage applied to the induction motor using a reactor or thyristor, which is divided into a Scherbius type and a Kramer type. The Scherbius type method is further divided into a motor-generator controlling type and an inverter controlling type.
In the motor-generator controlling type method, a secondary output of a wound-rotor type induction motor is rectified by a silicon rectifier into direct current and the output is supplied to a DC motor. The DC motor drives an induction generator and electricity generated by the induction generator returns to power. Thus, the motor-generator controlling method controls the speed of the induction motor through field magnet control of the DC motor.
According to the inverter controlling type method, a secondary output of a wound-rotor type induction motor is rectified by a silicon rectifier into direct current. The direct current is converted by a thyristor inverter into a three-phase alternating current, and the alternating current returns to power. That is, the inverter controlling type method controls the speed of the induction motor by phase-shifting the inverter.
One of the parameters affecting the speed control of an induction motor as above is a resistance component. That is, as temperature increases according to a prolonged operation of the induction motor, resistance values of a stator and a rotor of the induction motor vary and accordingly current values applied to the stator and the rotor change so that accuracy in speed control of a control system of the induction motor is lowered. Conventionally, to overcome the above problems, the resistance values of the stator and the rotor are estimated to be used in speed controlling.
FIG. 1 is a view schematically showing the structure of a system for estimating resistance values of a stator and a rotor of an induction motor which is adopted to a conventional method for estimating resistance values of a stator and a rotor of an induction motor.
Referring to FIG. 1, a system 100 for estimating resistance values of a stator and a rotor of an induction motor according to the conventional technology includes an all-dimension observation apparatus 101 for obtaining estimated values of current and magnetic flux, a speed estimating portion 102 for estimating is rotational speed utilizing an estimated value of magnetic flux obtained by the all-dimension observation apparatus 101 and the difference between the estimated value of current and actual current, and a motor constant estimating portion 103 for estimating resistance values of a stator and a rotor of an induction motor based on the values obtained by the all-dimension observation apparatus 101 and the speed estimating portion 102. Here, the above constituents are not common hardware comprising electrical parts or circuit elements but imaginary constituents presented as apparatuses to help in the understanding of an algorithm existing in a software of a computer system.
According to the conventional system for estimating resistance values of a stator and a rotor having the above structure, the voltage vector V.sub.s and current vector i.sub.s of the voltage applied to the stator during driving of the induction motor are measured and an estimated value i.sub.s of current of the stator and an estimated value .PHI..sub.r of magnetic flux of the rotor are obtained based on the above measured values. Here, the estimated value i.sub.s of current and the estimated value .PHI..sub.r of magnetic flux are obtained by the following Equation (1). ##EQU1##
Here, A and B having a relationship of A.ANG.R.sup.4.times.4 and B.ANG.R.sup.4.times.2 represent a system matrix of the induction motor and G having a relationship of G.ANG.R.sup.4.times.2 represents a gain matrix of the observation apparatus.
When the estimated value i.sub.s of current and the estimated value .PHI..sub.r of magnetic flux are obtained as above, the rotation speed is estimated by the speed estimating portion 102 using the estimated value .PHI..sub.r of magnetic flux and the difference (i.sub.s -i.sub.s) between the actual current i.sub.s and the estimated value i.sub.s of current. Here, the speed estimation value is obtained by the following Equation (2). ##EQU2##
Here, .omega..sub.r, K.sub..rho., K.sub.i and s represent speed estimate value, proportional constant, integral constant and Laplace operator, respectively.
When the speed estimate value .omega. is obtained as above, resistance values of the stator and the rotor are estimated after applying an AC component to the magnetic flux current of the induction motor by the motor constant estimating portion 103. Here, the estimated values of resistance of the stator and the rotor are obtained by the following Equation (3). ##EQU3## Here, R.sub.s, R.sub.r, L.sub.r, and M represent the estimated value of resistance of the stator, the estimated value of resistance of the rotor, inductance of the rotor, and mutual inductance, respectively. Also, K.sub..rho.1 and K.sub..rho.2 represent proportional constants and K.sub.i1 and K.sub.i2 represent integral constants.
However, the estimated value diverges in the case in which the induction motor operates in a generator mode. Also, the conventional method lacks general usability since it is applicable only when an estimation apparatus of a MRAS (model reference adaptive system) type of a sensorless speed control system is employed.